Charakterisierung des Ermüdungsverhaltens von metallischen Strukturwerkstoffen unter Einbeziehung der energiedispersiven Laue-Beugung

The microstructural processes that occur within cyclically stressed metallic components, which result in damage and sudden failure, remain poorly understood. In order to gain further insights, it is planned to use a special detector that is capable of measuring the energy of hard X-ray radiation with a high spatial resolution.
The objective of this research is to determine the mechanisms of damage and lattice strains in metallic structural materials subjected to cyclic stress using a novel measurement technique that integrates an energy-resolving detector with high-energy white X-rays. In order to achieve this objective, a series of dislocation arrangements were set in a range of polycrystalline materials, each exhibiting distinct dislocation glide behaviours. This was achieved by varying two key parameters: the amplitude of plastic strain and the number of cycles. Subsequently, the diffraction behaviour was correlated with the aforementioned dislocation arrangements. The diffraction patterns that are formed are dependent on the state of the internal stresses with respect to the geometry and the energy that is measured. The radially extended diffraction reflections are referred to as streaks.
In the example material pertaining to wavy glide behaviour, nickel, a correlation between the diffraction patterns and the dislocation arrangements is evident. In particular, individual streaks are identified as long as cell structures are observed which consist largely of random dislocation walls. These streaks can be evaluated with regard to the lattice strains. As the number of geometrically necessary dislocation walls increases in response to elevated cyclic loading, the evaluation process becomes more challenging due to the lateral displacement and the resulting overlap of the streaks.
In contrast, in the example material for planar glide behaviour, brass, the radiation is diffusely scattered by the high density of stacking faults and statistically stored dislocations, resulting in the absence of streaks that can be evaluated with regard to the lattice strains.
In a nickel-chromium alloy exhibiting mixed glide behaviour due to the presence of precipitates, the formation of streaks can be observed and correlated to a lattice strain. This indicates that there are few to no geometrically necessary dislocations present on the dislocation walls, which are formed in addition to the parallel dislocation segments. Moreover, the repetitive sliding processes of the dislocations to overcome the chromium carbides result in a reduction of the material’s strength and an overall decrease in the strains within the lattice.
The two-phase example material, duplex steel, exhibits individual clear streaks that can be attributed to the ferritic or austenitic phase and indexed accordingly. The alteration in the diffraction pattern observed in ferrite can be attributed to the clustering of dislocations, which correspond to random dislocation walls. In contrast, the change in the diffraction pattern in austenite is caused by an increase in statistically stored dislocations, stacking faults and twin grain boundaries. Due to the spatial restriction of dislocation arrangements to the vicinity of phase boundaries, lattice strains exhibit significant fluctuations.